Today's KNOWLEDGE Share : High shear layers in Injection molding process

Today's KNOWLEDGE Share

A local temperature increase in excess of 100°C is absolutely a realistic number for the thin high shear layers in Injection Molding.

Because this high T will last often less than a second (filltimes are short, the cold steel mold is extremely close), such high melt T will not necessarily result in a very visible quality loss, like discoloration.

This high shear-heating also happens in your barrel if you push RPM and backpressure up. However, testing for melt T plunging a thermocouple on the purged melt will hardly measure a significant change in average temperature, because the very hot layer is extremely thin (small overall volume fraction).

Despite the small volume involved in possible degradation, significant issues may arise, like VOC (volatile organic compounds) degassing, which will mess with your process and parts.

The message is : be aware that due to the low thermal conducivity of plastics, heating will be very localized (adiabatic) and will be difficult to evidence with typical observation/testing, while already creating issues.

Vito leo